1. Field of the Invention
The present invention relates to a method for saving power in a mobile communication system, and more particularly, to a technique for a power saving mode transition on a mobile terminal in radio packet communications.
2. Description of the Related Art
Conventionally, the mobile communication system such as PHS or HIPERLAN Type 2 (High Performance Radio Local Area Network) is provided with a power saving mode function for reducing a battery consumption of a mobile terminal.
For example, the cellular system such as PHS is a communication system of circuit switching type in which the end of communication is clear so that, after the communication is terminated, the mobile terminal makes a transition to a power saving mode for intermittently receiving signals from radio base stations. There is only one period for the intermittent receiving in this power saving mode called super-frame period (see ARIB RCR STD-28 second generation cordless telephone system standard specification).
Also, in the HIPERLAN Type 2, a transition to the power saving mode for intermittently receiving signals from access points (AP) is made in the case of power saving. In this power saving mode, at most sixteen types of the intermittent receiving period can be selected, with the maximum value of the intermittent receiving period being given by a time span of the broadcast frame at which all the terminals (mobile terminals) receive signals from the AP. These sixteen types of the power saving mode are provided in order to prevent the concentration of mobile terminals to one power saving mode, so that a negotiation between the AP and the mobile terminal is carried out to determine one intermittent receiving period from the sixteen types at a time of transition to the power saving mode, and the intermittent receiving is carried out by using one and only one determined intermittent receiving period during the power saving mode such that the intermittent receiving period is not changed dynamically (see, ETSI TS 101 761-2 V1.1.1 ETSI BRAN HIPERLAN Type 2 Technical Specification).
However, in the conventional art described above, during the power saving mode, the mobile terminal intermittently receives signals from the radio base stations or APs and recovers the communication state when the signals are received, so that in the case of making a transition to the communication state, a delay that is at most as much as the intermittent receiving period occurs since the signals are sent from the radio base station (AP) until the communication state is recovered. For this reason, when a transition to the power saving mode is made during the realtime communications, a paging delay occurs and the packet receiving is not carried out normally so that there is a possibility for the quality degradation due to the packet loss.
In particular, in the cellular system described above, only one intermittent receiving period during the power saving mode is set up so that the intermittent receiving period must be reduced in accordance with the realtime communications when the realtime and non-realtime packets coexist, but the reduction of the paging delay requires the corresponding reduction of the intermittent receiving period so that there has been a problem that the sufficient effect cannot be obtained from a viewpoint of the power saving.
On the other hand, in the HIPERLAN Type 2 described above, a plurality of intermittent receiving periods during the power saving mode exist, but a value selected at a time of the transition to the power saving mode is continually used as the intermittent receiving period of the mobile terminal so that it is impossible to cope with the change of the communication state and there has been a problem that the sufficient power saving effect cannot be realized.